He was the star of the youth team, which contained David Beckham and Paul Scholes and tipped for stardom, but one challenge in a reserve match ended it all.
Sir Alex Ferguson was considering selecting the then 18-year-old for an FA Cup semi-final days later, but in April 1994 he turned out for a second string game against Blackburn.
A challenge on the youngster by defender Nicky Marker was so bad that Thornley ruptured his MCL, and ACL, medial capsule and detached his meniscus.
The trauma to his knee was so great that he was never able to realise his potential and despite making 13 appearances for the 'Red Devils', he was limited to a career in the lower leagues.
A far cry from the prediction of World Cup winner Nobby Stiles who said: ''Ben is the closest I've seen to George Best in all my time at the club.''
A Book titled 'Tackled: The Class of 92 Star Who Never Got To Graduate' details his rise and fall, in which his team mates acknowledge his talent.
David Beckham says: ''Ben would have outdone us all - that's the sad part.''
Scholes says Thornley was ''a step above all of us, he could do everything.'' Gary Nevile says he was ''one of the most outstanding talents I ever played with.''
Thornley ultimately fell into a cycle of depression and alcoholism after his career never lived up to expectations and while he is recovering now his life remains a story of what could have been.